To vary a transmission ratio between input and output speeds continuously, a CVT of a belt type varies a so-called pulley ratio by varying a radius of contact point between a belt and a pulley. In the case of the belt type CVT because of its nature, it is considered undesirable to provide a means, such as a one way clutch, for limiting the direction of torque transmission as provided in an ordinary automatic transmission composed of a torque converter and a gear system. To vary the pulley ratio, the belt type CVT is arranged to vary the width of a pulley groove formed between a fixed pulley disc and a movable pulley disc by shifting the movable disc relative to the fixed pulley disc by the action of a fluid pressure such as an oil pressure like a piston.
In the transmission of this type, in general, a controller is provided to control the transmission ratio in response to predetermined one or more operating parameters such as the vehicle speed, throttle opening and engine speed, according to a predetermined shift pattern within a transmission ratio control region corresponding to each shift position. The transmission ratio is controlled within a D range transmission ratio control region (or between D range ratio limits) when a normal drive range (such as a D range) is selected, and within an engine brake range transmission ratio region (or between engine brake range ratio limits) when the driver selects an engine brake range (such as 2 range, Ds range, L range or 1 range) which can provide an engine braking effect and which has a minimum transmission ratio greater than the minimum transmission ratio of the normal drive range. When the accelerator pedal is released and the throttle opening is decreased, the controller of this type tends to hold the transmission ratio at a predetermined ratio continuously independent of the actual engine revolving condition. Specifically in a coasting state in which the throttle opening is small and the vehicle speed is relatively high, the transmission ratio is continuously held at the minimum ratio.
On a dry paved road having a high friction coefficient .mu., while releasing the accelerator pedal with the intention of decelerating the vehicle by engine braking, the driver may change the shift position from the D range to the engine brake range such as the L range or the 2 range. Before this manual downshift, the transmission ratio is at the minimum ratio of the D range because of the reduction of the throttle opening. Accordingly, the transmission ratio is increased forcibly and abruptly from the minimum ratio of the D range to the minimum ratio of the engine brake range. With this ratio change, a back torque of the engine, or engine brake, is applied, as a braking force, on the drive wheels. If this occurs during a downhill operation, the gravitational acceleration acts on the vehicle body in the direction opposing the decrease of the vehicle body speed (or the vehicle speed), and the transmission ratio is held in the forced state for a relatively long time. During this, therefore, the revolution speed of the input shaft of the CVT varies widely, causing a transient fluctuation of the inertia torque, and there arises the possibility of slip between the belt and pulley.
To prevent such a slip of the belt, a Japanese Patent Provisional Publication S4(1992)-203665 proposes a CVT control system which senses an acceleration acting on the vehicle body, and which, if the sensed acceleration exceeds a predetermined level, restrains the rate of change of the transmission ratio due to the manual downshift from the D range to the engine brake range. This system prevents an abrupt change of the transmission ratio in this way, and thereby prevents the belt slip.